Using force testing for ACL rehab and return-to-sport
Ever wonder how technology can help you return to sport?
Force can be defined as a push or pull on an object (or person) that causes a change in motion. Most athletic movements involve an individual exerting force against the ground or an object. This makes it a useful measurement to assess when looking at someone’s physical abilities.
Examples of force include:
1.) A sprinter applying force into the ground to propel themselves forward
2.) A Powerlifter applying force to lift a barbell off the ground during a deadlift.
There are different metrics that can measure someone’s ability to apply force and understanding these metrics is incredibly useful to make an informed return to sports decisions.
Two metrics we will break down in this article and apply to a return to sport ACL case study are peak force and impulse.
Peak force: measures the maximum force that someone can produce
Impulse: Allows us to understand how they ramp up to that maximum force and then sustain it over a period of time.
We find that peak force and impulse tend to provide more insight compared to other common measures of strength and power. This allows us to better choose exercises and create a specific individualized program.
When assessing an individual, it's important that we look at these metrics in conjunction with other assessments and understand the specific demands of the sport that the person participates in. Peak force or impulse in isolation don’t tell you enough to make informed decisions and create a well-rounded plan.
Someone may have high peak force numbers, but there is no guarantee that they will be able to produce that force quickly in the context of their sport. On the flip side, someone may be able to ramp up to maximum force quickly, but their peak force is so low that it limits their performance since they don’t produce enough force.
We look at them to make sure someone has certain physical capacities that they can utilize in training and rehabilitation to successfully improve performance and reduce risk of injury.
When assessing this data involving peak force and impulse, we can look at it in terms of total body OR we can compare the left to the right side to determine any asymmetries.
This becomes useful in post-operative scenarios or when rehabbing an injury because it tells us what qualities need to be focused on to restore limb symmetry and train well. We can then reduce the risk that someone reinjures themself or runs into other issues when returning to activity.
Looking at the left versus right side, specifically, it is important that we focus on restoring symmetry. In terms of these two metrics. We can improve someone's peak force abilities on one limb (think more traditional strength-based training) OR we can improve someone’s ability to produce and sustain force within a given period of time (think more power, plyometrics, jump training).
One test we use in our SOHO Physical Therapy clinic to measure peak force and impulse is called an isometric mid-thigh pull test (IMTP).
The IMTP is performed with an individual standing on force plates, with a barbell equidistant between their hips and knees. From there, the barbell is locked in place and the athlete pulls the barbell vertically in a maximum effort to determine how much force he/she can produce and how they ramp up and sustain that force. (see video here and client demo here)
This is an important test for us, especially in lower extremity post-operative or return to sport scenarios. It provides us information on how an athlete’s peak force and impulse are improving over time. The data also lets us know what to emphasize in someone’s training program from a strength and power standpoint. From a research standpoint, the test is reliable and easy to replicate (1).
Here’s an example of an individual's IMTP at 11 months post-operative left ACLR and again at 12 months. We saw this individual 2x/week on a training program that emphasized strength work with power and plyometrics included as well.
The graph shows us the force produced by this individual over a period of time.
11 months: Asymmetrical force production(looking at the gap between the orange and blue line)
12 months: Improvement in overall force production and also improvements in symmetry between both sides.
Green line: represents the total vertical force of the left and right side combined
Orange line: represents only the right leg
Blue line: represents only the left leg.
When interpreting the graph, peak force is going to be the highest point on the graph for each color line. It will be a different number for green, orange, and blue meaning the total peak force will differ for the left leg peak force compared to the right leg peak force. Impulse is how that force is applied over time.
The way we measure this is the area under the curve or how much space there is beneath one of the lines in each time frame. Impulse can be a bit trickier to understand, but for the purpose of this article, it is telling us how force output is increasing, decreasing, or remaining relatively the same for a given period.
Some key takeaways from the one-month difference:
Peak force on both legs combined (green line) increased showing an improvement in maximum force production capabilities. It is a little above 2000N at 11 months and closer to 2400N at 12 months (see the highest point on the green line).
Peak force asymmetry between left and right decreased. There is less of a gap between the orange and blue lines in the 12-month test at their highest points.
There is still a discrepancy in impulse between the left and right. The right-side ramps up faster than the left and sustains that for most of the pull. You can see this by how steep the orange line is compared to the blue.
While this test and these metrics alone don’t provide us with a full picture of progress, they are a good indicator on if a person's limb symmetry is improving and if they are closer to returning to sport.
Additionally, we perform isolated strength testing and jump testing to stay consistent with the research on post op ACL - stating that > 90% symmetry in our IMTP test measures along with certain isolated strength numbers and jump numbers ensure a safer return to sport (2).
This provides us with a full assessment of objective criteria to reduce future risk for injury and it allows us to understand how these metrics improve over time to guide our decision-making going forward.
For this specific individual, we can see that his strength numbers (peak force) have improved, but he is still struggling to ramp up force quickly on his post-operative leg (discrepancy in impulse).
In his next phase of training we plan to include exercises that emphasize the production of force quickly so as to return symmetry for that specific quality, and in turn, allow him to be quicker and more explosive off of that leg.
Why is this important for Physical Therapy?
Having a set of objective criteria is incredibly important for post-operative rehabilitation to ensure individuals are hitting their rehab goals and are prepared for the demands of what they plan to return to. This article shows how we at Moment PTP use objective data and how it informs our decision-making to set our clients up for success.
If you’re an athlete, whether in the NBA or at your local rec league you should be using force plate testing as you return to your sport. If you’re struggling to reach your performance goals, our SoHo Physical Therapy Clinic can help.
If you’re looking to learn more about sports rehab and how we help patients return to sports? Click here.
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